Most crawl spaces have foundation vents to prevent buildup of excess moisture. But vented crawls often become breeding grounds for mold and moisture, leaving framing rotted and insulation soaked. Now many building scientists recommend sealing and insulating these spaces, and there's proof behind their argument.

HIGH AND DRY: Building scientists increasingly favor sealing and insulating crawl spaces, saying foundation vents fail to keep out mold and moisture. How best to seal depends on your area's climate, but all crawl spaces need a good moisture barrier. Creating that includes taping seams in the polyethylene moisture barrier and extending it partway up concrete block columns and foundation walls. Photo: Courtesy Advanced Energy In a recently published study, researchers at Advanced Energy (AE) of Raleigh, N.C., described how they sealed and insulated crawl spaces in 12 to 15 homes in three different regions: hot, humid Baton Rouge, La.; cold, dry Flagstaff, Ariz.; and temperate Princeville, N.C. Their work from 2005 through 2008 confirmed that a closed crawl will stay drier in any climate–and will cut energy bills if built with climate-appropriate detailing.

Researchers sealed the crawl in each home as carefully as if it were living space. They weather-stripped the access door, and blocked or foam-sealed penetrations in the foundation and floor framing. They covered foundation walls and dirt floors with a polyethylene moisture barrier, and installed an undersized HVAC supply duct with just enough airflow to evaporate any moisture that got past the seals.

Four homes in each area got standard floor insulation–R-19 fiberglass batts between the joists–while in four more AE replaced the batts with R-13 polyisocyanurate rigid foam fastened to the foundation walls. In Baton Rouge, an extra three homes got wall insulation so researchers could measure the difference between those whose HVAC ducts ran through the attic and those whose ducts ran through the crawl. Each group also included four control homes with vented crawls. Data loggers measured crawl space humidity for more than a year in Princeville and Baton Rouge and for one heating season in Flagstaff. AE also monitored home energy bills.

The result? All of the closed crawls stayed drier than the controls, regardless of insulation or duct placement.

In Princeville, the closed-crawl homes used 15% to 18% less energy than the controls. In Flagstaff, homes with floor insulation used 20% less energy than the control houses while those with foundation wall insulation used 53% more. Cyrus Dastur, the project's head researcher, believes that in cold climates, floor insulation can prevent radiant heat loss to the cold ground below.

Results in Baton Rouge ranged from a 6% savings to a 29% penalty. In homes with HVAC ducts in the attic, those with wall-insulated crawls only performed better than the control homes for part of the year. Homes with ducts in wall-insulated closed crawls used less energy all year.

The Flagstaff homes' crawls had to be re-opened after the study because of high radon levels. In areas known for radon, it's a good idea to install soil gas exhaust piping into the crawl space during construction.

As for water, a closed crawl should have a moisture meter paired to an interior alarm, and the floor should be sloped to an emergency drain or sump pump. If there is a leak, a properly detailed closed crawl should be able to handle it. A contractor in North Carolina had to clean up a closed crawl that had been flooded by a burst pipe. After removing the water, the tempering duct dried the space within an hour.

Dastur says most codes requiring crawl space ventilation are based on research done in the early 1900s, way before affordable impermeable ground covers came out. In addition, the cost of vents is negligible, while closed systems can cost $2 to $3 per square foot in new construction.

At AE's urging, North Carolina's building code now gives equal standing to closed and open crawls, but closed crawls are still viewed as an exception by the International Residential Code.

–Charles Wardell is a freelance writer based in Tisbury, Mass.

This is the first in a series examining new thinking about how to build homes better, and sometimes help homes qualify for green building status.